The U joints are fine. Would it be obvious if there was a rock clanking up in there. if not how would check by a jack stand. If the drive belt was bad would it have this affect. if so how would i get to it and how would i know if it needed to be replaced. i also dont know if that rock theory would cause the whole entire machine to jerk around like that. It almost seems as though it is a tranny problem.(i hope not). It only does it in very low speeds and while braking. its defiatly not the brakes.(im pretty sure). if its jerking at a low speed in reverse and forward could it be bad gas or a fouled plug.

Need more answers!!!